1. Field of the Invention
The present invention relates to a photodetector of a waveguide type suitable for use in an optical integration circuit and the like, a method of fabricating such a photodetector and an optical communication system wherein such a photodetector is used for receiving a signal light.
2. Related Background Art
FIG. 1 shows a prior art waveguide type photodetector wherein a waveguide structure and a light detecting unit are combined (see U.S. Pat. No. 4,360,246). In FIG. 1, an undoped (.phi.-) Al.sub.y Ga.sub.1-y As first clad layer 102 having a thickness of 0.5 .mu.m-1 .mu.m, an undoped Al.sub.x Ga.sub.1-x As waveguide layer 103 having a relatively large thickness of 2 .mu.m-5 .mu.m and an n-type GaAs active or light absorbing layer 104 having a thickness of 0.1 .mu.m-1 .mu.m are successively layered on a semi-insulating GaAs substrate 101. Here, the relation between the mole fractions x and y is set to x&lt;y in order to make the refractive index of the first clad layer 102 smaller than that of the waveguide layer 103.
The n-type GaAs light absorbing layer 104 is formed in an island shape on the .phi.-Al.sub.x Ga.sub.1-x As waveguide layer 103 by using a selective etching of GaAs and AlGaAs to expose the surface of the .phi.-Al.sub.x Ga.sub.1-x As waveguide layer 103. Further, an insulating material 105 is formed in a stripe shape on the .phi.-Al.sub.x Ga.sub.1-x As waveguide layer 103 to form a three-dimensional waveguide 110. The refractive index of the insulating material 105 such as a silicon dioxide should be larger than that of air. Thus, the equivalent refractive index is made different between the waveguide 110 and portions other than the waveguide 110 to achieve a light confinement in a lateral direction in the waveguide layer 103.
The operation of this device will be described. A signal light 114 enters an end surface 112 of the device and is propagated through the waveguide layer 103. The propagated light is confined by the .phi.-Al.sub.y Ga.sub.1-y As first clad layer 102 acting as a lower clad layer and the insulating material 105 acting as an upper clad layer and reaches the n-GaAs layer 104 at a position indicated by an A--A' line. The refractive index of the n-GaAs layer 104 is larger than that of the waveguide layer 103, and therefore, the propagated light leaks out from the waveguide 110 to the n-GaAs layer 104. The leaking light is absorbed by the n-GaAs layer 104, and electrons and holes are generated therein. The light detecting unit in this prior art device has a field effect transistor (FET) structure including a source electrode 106, a drain electrode 107 and a gate electrode 108. The former two electrodes 106 and 107 are respectively ohmic electrodes while the latter one electrode 108 is a Schottky electrode. Therefore, the electrons are taken out by the ohmic electrodes 106 and 107 formed on the light absorbing layer 104, and the light detection is thus performed. The holes are taken out by the gate electrode 108. The gate electrode 108 functions to generate a depletion layer in the light absorbing layer 104 and thus controls a dark current and a drain current.
In the prior art device, however, the photon energy E.sub.s of the input signal light 114 or propagated light must be in the following range since the light 114 is to be propagated through the .phi.-Al.sub.x Ga.sub.1-x As waveguide layer 103: EQU 1.42&lt;E.sub.s &lt;1.42+1.247x(eV).
When the calculaton is performed by substituting the numerals of x disclosed in the above USP into this formula and the photon energy E.sub.s is converted to the wavelength, the wavelength is found to be in a range between 0.64 .mu.m and 0.87 .mu.m. If a mode calculation of the waveguide 110 is conducted on the basis of this wavelength range, the waveguide 110 turns out to have a multi-mode. The propagated light will have a higher order mode and its speed will be slow when the waveguide has a multi-mode. As a result, compared an electric signal output from the photodetector with an input signal thereinto, the output signal has a wave shape with a dull rise. Further, an S/N ratio is lowered.
Furthermore, it is difficult to make the thickness of the mixed crystal uniform, and the lateral confinement of the three-dimensional waveguide 110 is weak since this confinemnet is performed only by utilizing the difference in the refractive index between the insulating material 105 and the air. Moreover, the absorption edge of the waveguide 110 is dull since the waveguide 110 is formed using the mixed crystal, and therefore, the wavelength of the input signal light 114 should be considerably longer than the absorption edge.
The width of the absorption layer 104 should be less than or at most a little larger than that of the waveguide 110 for the propagated light must be three-dimensionally guided with a little expansion also under the light detecting portion. Therefore, a fine working process should be used when the device is fabricated.